ZWO ASI178MM-cooled UV-imaging

One of the areas of photography that I have an interest in, and wanted to test out of the ASI178MM-cooled for a while, is UV-imaging. That is imaging below 400nm wavelength, which is  invisible to normal human vision. Now it is unclear the response of the IMX178 Sony Starvis sensor to the light below 400nm, but I felt I needed to test this out, particularly as it does not have a Bayer Matrix to affect UV-transmission. But the big question I had was whether the AR window protecting the sensor would block deeper UV?

Now ZWO do provide the relative response of this sensor from 400nm to infrared range on their website. This suggests that it should still be responsive to the light below 400nm.

Quantum efficiency curve for IMX178
Quantum efficiency curve for IMX178 – from ZWO website

Equipment used: ZWO ASI178MM-cooled camera, 17-31mm M42-M42 focusing helicoid, T2-Nikon F adapter mounted on Novoflex ASTAT-MFT adapter with Arca plate, Coastal Optics 60mm F4 APO lens, Microsoft Surface Pro 3 using Sharpcap to capture images.

Below was the setup I used for capturing the images of a home-grown yellow marigold.

ZWO ASI178MM-cooled imager connected via M42-M42 focusing helicoid, T2-Nikon F adapter, with Novoflex ASTAT-MFT adapter, Coastal Optics 60mm F4 APO lens
ZWO ASI178MM-cooled imager connected via M42-M42 focusing helicoid, T2-Nikon F adapter, with Novoflex ASTAT-MFT adapter, Coastal Optics 60mm F4 APO lens
Yellow Marigold from my garden
Yellow Marigold from my garden

I chose to use F4 with the Coastal Optics, as the crop factor of this camera is 4.8x so the depth of field should be more than with other cameras particularly as the camera has to be a significant distance away from the subject for it to fit in the field of view.

First image was shot with Baader U, using about 1.3s exposure so I could manually burst the Quantum X4D flash once. Notice the bright UV-reflective petals with some UV-dark lines running through them, as well as the dark target center of the flower – no IR-leak here.

Shot with Baader U @ F4, Gain 0, 1/8 power from Quantum X4D
Shot with Baader U @ F4, Gain 0, 1/8 power from Quantum X4D

The second image was taken using the 330AF20 filter, with burst of light from Quantum X4D as above. Notice that the center is still UV-dark, so there is no IR-leak visible in the image, and as this filter only transmits between 320-340nm, it is clear that even with the AR window of the imager that deeper UV is still getting through.

Shot with 330AF20 filter @ F4, Gain 0, 1/8 power from Quantum X4D
Shot with 330AF20 filter @ F4, Gain 0, 1/8 power from Quantum X4D

Conclusion:

If you imagine that there are people who have taken great lengths to remove the Bayer Matrix from their camera for UV-imaging. Now imagine that the ZWO ASI178MM-cooled is a mono camera built for astrophotography, microscopic photography, and also can be used for UV-imaging, as it is very sensitivity to UV (due to lack of Bayer Matrix).

Looking at the images I have managed to capture in UV, it would suggest that the AR window does allow UV through below 340nm, as the 330AF20 only transmits between 320-340nm, with peak at 330nm; otherwise I would have needed to buy a Fused Silicate or Quartz window to replace the AR window.

And with the gain available with this camera to boost the response to the photons, being able to go up to a maximum gain of 510 (although I think 100-200 would suffice), it should have great potential for UV-imaging. If only there was a way of capturing images without a computer – perhaps just using an iPhone or tablet, now that would be a game changer.

Boon